Multiple sclerosis (MS) is an inflammatory-mediated demyelinating disease of the central nervous system (CNS). While historically considered to be a disease of white matter (WM), recent studies have estimated that cortical demyelination exceeds WM demyelination in many MS patients. While cortical demyelination is a major aspect of MS pathogenesis, little is known about the dynamics and mechanisms of cortical myelin loss. This is especially true for subpial lesions, which are the most abundant type of cortical demyelination. Subpial lesions fail to show many of the pathological hallmarks of WM lesions, with little infiltration of peripheral immune cells or breakdown of the blood- brain barrier. Data also support the concept that cortical atrophy/demyelination correlates more strongly with clinical and cognitive disability than WM atrophy/demyelination. Studies outlined in this proposal will investigate how subpial demyelination occurs in MS and investigate the influence of brain WM demyelination on WM MRI alterations and brain atrophy. Specific Aim 1 will test the hypotheses that a dying-back oligodendrogliopathy occurs in upper layers of the cerebral cortex with age and activates microglia, which subsequently remove defective myelin. Loss of oligodendrocytes and myelin induces new oligodendrocyte production and remyelination. In MS brains, this oligodendrogliopathy is increased and an imbalance between demyelination and remyelination results in subpial lesions. Studies in Specific Aim 1 will also establish the turnove of oligodendrocyte lineage cells by measuring the level of `nuclear bomb' 14C in oligodendrocyte lineage cells. Specific Aim 2 will establish the molecular phenotype of microglia isolated from MS cortex and determine whether microglia cells are removing defective subpial myelin in MS brains. Since current anti-inflammatory therapies may not stop subpial demyelination, these studies are highly significant because they may identify novel therapeutic targets that will reduce or eliminate subpial demyelination. Specific Aim 3 investigates a cohort of 18 postmortem MS cases that have significant brain WM MRI lesion loads, but virtually no brain WM demyelination. Preliminary studies detected significant spinal cord and subpial cortical demyelination and provided pathological confirmation of MS. We will ask whether brain WM demyelination is essential for brain WM MRI abnormalities, brain WM atrophy, and cortical atrophy and identify other pathological changes that may contribute to these surrogate markers of MS disease progression. These studies have the potential to refine dogmas of MS pathogenesis.